This is an actual effect used in some dance performance videos, especially kpop! Yes, they slow down the music and perform at the slower tempo before syncing it up in post. But it usually gives a sort of stop motion effect in the process that I didn’t notice when watching the performance in RRR. I honestly do not think they slowed down the performance in any way on set. I’m wondering if they show it at a higher frame rate to give it a motion blur effect?